Life and death of a sewage treatment plant recorded in a coral skeleton δ 15 N record.

We investigated the potential of coral skeleton δ15 N (CS-δ15 N) records for tracking anthropogenic-N sources in coral reef ecosystems. We produced a 56yr-long CS-δ15 N record (1958-2014) from a reef flat in Guam that has been exposed to varying 1) levels of sewage treatment 2) population density, and 3) land use. Increasing population density (from <30 to 300ind·km-2 ) and land use changes in the watershed resulted in a ~1‰ enrichment of the CS-δ15 N record until a sewage treatment plant (STP) started operation in 1975. Then, CS-δ15 N stabilized, despite continued population density and land use changes. Based on population and other considerations, a continued increase in the sewage footprint might have been expected over this time. The stability of CS-δ15 N, either contradicts this expectation, or indicates that the impacts on the outer reef at the coring site were buffered by the mixing of reef water with the open ocean.